The US Supreme Court on Tuesday delivered a setback to President Bush's expansive vision of presidential power, ruling that a unilateral attempt by Mr. Bush to order state courts to comply with an international treaty violated "first principles" of constitutional government.

"The President's authority to act, as with the exercise of any governmental power, must stem either from an act of Congress or from the Constitution itself," wrote Chief Justice John Roberts in the majority opinion.

The court ruled 6 to 3 that the president overstepped his authority when he ordered the Texas judiciary to abide by a 2004 ruling of the International Court of Justice (ICJ) mandating a new hearing for a Texas death row inmate and 50 other Mexican nationals.

At issue was whether Jose Medellin, a Mexican citizen on death row in Texas, should receive a special hearing to determine whether his right to a fair trial had been violated because he was unable to consult with Mexican consular officials after his 1993 arrest.

The Texas courts determined that Mr. Medellin had received all the fair process he was due during his trial and appeals. But Bush, citing the ICJ ruling, issued a presidential memorandum directing the Texas courts to give Medellin a new hearing. The Texas judges refused.